The momentary disappearance of the clock signal that accompanies data is a major problem in the case of a synchronous transmission of digital data.
This phenomenon of clock signal loss appears especially when the link is set up by RF means in an electromagnetic environment disturbed by other transmissions. It can also appear in other types of links, for example wire links, especially as a consequence of line-coupling phenomena.
In the case of synchronous transmission, the loss of the clock signal generally leads to the loss of the associated data, as well as to the implementation of an often lengthy procedure for the general re-synchronization of the equipment. General re-synchronization procedures, when excessively frequent, result in a considerable reduction of the information transmission bit rate. The invention described and claimed in the present document deals with the way to resolve the problem posed by the momentary disappearance of the clock signal that accompanies the data in synchronous transmission. It is aimed at eliminating the operations of general re-synchronization, or at least spacing them out to the greatest possible extent.
In the context of the synchronous transmission of data (hereinafter called “synchronous transmission” for the sake of simplicity) the transmitter equipment simultaneously transmits payload data and a sequencing clock signal which the receiver equipment can use to sample and process the data satisfactorily. However, in certain circumstances, the sequencing clock signal may be highly disturbed. It may also completely disappear. In these circumstances, the receiver equipment generally uses a locally produced substitute clock signal. For the link to be maintained, the substitute clock signal must be as close as possible to the missing sequencing clock signal. Consequently, and if the loss of clock signal is brief, there is no loss of information bits. When the sequencing clock signal given by the transmitter equipment reappears, the receiver equipment once again uses the sequencing clock signal. The substitute clock signal is generally synthesized by the receiver element of the reception equipment, which is the upstream element in the reception chain. The clock signal substitution is generally done automatically without the downstream elements being informed.
In the most favorable cases, the substitute clock signal is a clock signal having the same frequency and the same stability as the sequencing clock signal coming from the transmitter equipment. The substitute clock signal may furthermore have been synchronized with the sequencing clock signal at a preliminary reset stage, for example when setting up a link or again during a general re-synchronization operation. However, even in this case, a drift appears over time between the transmitted clock signal and the local clock signal. In the event of the loss of the sequencing clock signal, this de-synchronization renders the sampling of the data through the local clock signal inefficient and swiftly leads to the loss of data bits.
The loss of data bits destroys the integrity of the data transmitted, generally causing the loss of complete frames, or even the totality of the transmitted message. It necessarily leads to the implementation of a procedure of re-synchronization as soon as the transmitted clock signal is again detected by the receiver equipment.
The phenomenon is of course aggravated and the loss of integrity is even faster if the local clock signal is of lower precision.
One approach to resolving this problem of de-synchronization consists in implanting a reference clock in the transmitter equipment and in the receiver equipment and regularly synchronizing the reference clocks. However, this solution is difficult to implement in practice. It necessitates, for example, the use of remotely re-synchronizable devices and proves to be costly in time and indiscreet.